
                              How to hack TinyScheme
                              ----------------------

     TinyScheme is easy to learn and modify. It is structured like a
     meta-interpreter, only it is written in C. All data are Scheme
     objects, which facilitates both understanding/modifying the
     code and reifying the interpreter workings.

     In place of a dry description, we will pace through the addition
     of a useful new datatype: garbage-collected memory blocks.
     The interface will be:

          (make-block <n> [<fill>]) makes a new block of the specified size
               optionally filling it with a specified byte
          (block? <obj>)
          (block-length <block>)
          (block-ref <block> <index>) retrieves byte at location
          (block-set! <block> <index> <byte>) modifies byte at location
     
     In the sequel, lines that begin with '>' denote lines to add to the
     code. Lines that begin with '|' are just citations of existing code.

     First of all, we need to assign a typeid to our new type. Typeids
     in TinyScheme are small integers declared in an enum, very close to
     the top; it begins with T_STRING. Add a new one at the end, say
     T_MEMBLOCK. There can be at most 31 types, but you don't have to
     worry about that limit yet.

|    ...
|      T_PORT,
|      T_VECTOR,          /* remember to add a comma to the preceding item! */
|      T_MEMBLOCK
}     };

     Then, some helper macros would be useful. Go to where isstring() and
     the rest are defined and define:

>    int ismemblock(pointer p)      { return (type(p)==T_MEMBLOCK); }

     This actually is a function, because it is meant to be exported by
     scheme.h. If no foreign function will ever manipulate a memory block,
     you can instead define it as a macro

>     #define ismemblock(p) (type(p)==T_MEMBLOCK)

     Then we make space for the new type in the main data structure:
     struct cell. As it happens, the _string part of the union _object
     (that is used to hold character strings) has two fields that suit us:

|         struct {
|              char   *_svalue;
|              int   _keynum;
|         } _string;

     We can use _svalue to hold the actual pointer and _keynum to hold its
     length. If we couln't reuse existing fields, we could always add other
     alternatives in union _object.

     We then procede to write the function that actually makes a new block.
     For conformance reasons, we name it mk_memblock

>     static pointer mk_memblock(scheme *sc, int len, char fill) {
>          pointer x;
>          char *p=(char*)sc->malloc(len);
>
>          if(p==0) {
>               return sc->NIL;
>          }
>          x = get_cell(sc, sc->NIL, sc->NIL);
>
>          typeflag(x) = T_MEMBLOCK|T_ATOM;
>          strvalue(x)=p;
>          keynum(x)=len;
>          memset(p,fill,len);
>          return (x);
>     }

     The memory used by the MEMBLOCK will have to be freed when the cell
     is reclaimed during garbage collection. There is a placeholder for
     that staff, function finalize_cell(), currently handling strings only.

|     static void finalize_cell(scheme *sc, pointer a) {
|          if(isstring(a)) {
|               sc->free(strvalue(a));
|          }
>          else if(ismemblock(a)) {
>               sc->free(strvalue(x));
>          }
|     }

     There are no MEMBLOCK literals, so we don't concern ourselfs with
     the READER part (yet!). We must cater to the PRINTER, though. We
     add one case more in printatom().

|     } else if (iscontinuation(l)) {
|          p = "#<CONTINUATION>";
>     } else if (ismemblock(l)) {
>          p = "#<MEMORY BLOCK>";
|     }

     Whenever a MEMBLOCK is displayed, it will look like that.
     Now, we must add the interface functions: constructor, predicate,
     accessor, modifier. We must in fact create new op-codes for the virtual
     machine underlying TinyScheme. There is a huge enum with OP_XXX values.
     That's where the op-codes are declared. For reasons of cohesion, we add
     the new op-codes right after those for vectors:

|   OP_VECSET,
>   OP_MKBLOCK,
>   OP_MEMBLOCKP,
>   OP_BLOCKLEN,
>   OP_BLOCKREF,
>   OP_BLOCKSET,
|   OP_NOT,

     We add the predicate along the other predicates:

|   OP_VECTORP,
>   OP_BLOCKP,
|   OP_EQ,

     Op-codes are really just tags for a huge C switch, only this switch
     is broke up in a number of different opexe_X functions. The
     correspondence is made in table "dispatch_table". There, we assign
     the new op-codes to opexe_2, where the equivalent ones for vectors
     are situated. We also assign a name for them, and specify the minimum
     and maximum arity. INF_ARG as a maximum arity means "unlimited".

|     {opexe_2, "vector-set!", 3, 3},    /* OP_VECSET */
>     {opexe_2, "make-block", 1, 2},     /* OP_MKBLOCK */
>     {opexe_2, "block-length", 1, 1},   /* OP_BLOCKLEN */
>     {opexe_2, "block-ref", 2, 2},      /* OP_BLOCKREF */
>     {opexe_2, "block-set!",3 ,3},      /* OP_BLOCKSET */

     The predicate goes with the other predicates, in opexe_3.

|     {opexe_3, "vector?", 1, 1},  /* OP_VECTORP, */
>     {opexe_3, "block?", 1, 1},   /* OP_BLOCKP, */

     All that remains is to write the actual processing in opexe_2, right
     after OP_VECSET.

>     case OP_MKBLOCK: { /* make-block */
>          int fill=0;
>          int len;
>
>          if(!isnumber(car(sc->args))) {
>               Error_1(sc,"make-block: not a number:",car(sc->args));
>          }
>          len=ivalue(car(sc->args));
>          if(len<=0) {
>               Error_1(sc,"make-block: not positive:",car(sc->args));
>          }
>
>          if(cdr(sc->args)!=sc->NIL) {
>               if(!isnumber(cadr(sc->args)) || ivalue(cadr(sc->args))<0) {
>                    Error_1(sc,"make-block: not a positive number:",cadr(sc->args));
>               }
>               fill=charvalue(cadr(sc->args))%255;
>          }
>          s_return(sc,mk_memblock(sc,len,(char)fill));
>     }
>
>     case OP_BLOCKLEN:  /* block-length */
>          if(!ismemblock(car(sc->args))) {
>               Error_1(sc,"block-length: not a memory block:",car(sc->args));
>          }
>          s_return(sc,mk_integer(sc,keynum(car(sc->args))));
>
>     case OP_BLOCKREF: { /* block-ref */
>          char *str;
>          int index;
>
>          if(!ismemblock(car(sc->args))) {
>               Error_1(sc,"block-ref: not a memory block:",car(sc->args));
>          }
>          str=strvalue(car(sc->args));
>
>          if(cdr(sc->args)==sc->NIL) {
>               Error_0(sc,"block-ref: needs two arguments");
>          }
>          if(!isnumber(cadr(sc->args))) {
>               Error_1(sc,"block-ref: not a number:",cadr(sc->args));
>          }
>          index=ivalue(cadr(sc->args));
>
>          if(index<0 || index>=keynum(car(sc->args))) {
>               Error_1(sc,"block-ref: out of bounds:",cadr(sc->args));
>          }
>
>          s_return(sc,mk_integer(sc,str[index]));
>     }
>
>     case OP_BLOCKSET: { /* block-set! */
>          char *str;
>          int index;
>          int c;
>
>          if(!ismemblock(car(sc->args))) {
>               Error_1(sc,"block-set!: not a memory block:",car(sc->args));
>          }
>          if(isimmutable(car(sc->args))) {
>               Error_1(sc,"block-set!: unable to alter immutable memory block:",car(sc->args));
>          }
>          str=strvalue(car(sc->args));
>
>          if(cdr(sc->args)==sc->NIL) {
>               Error_0(sc,"block-set!: needs three arguments");
>          }
>          if(!isnumber(cadr(sc->args))) {
>               Error_1(sc,"block-set!: not a number:",cadr(sc->args));
>          }
>          index=ivalue(cadr(sc->args));
>          if(index<0 || index>=keynum(car(sc->args))) {
>               Error_1(sc,"block-set!: out of bounds:",cadr(sc->args));
>          }
>
>          if(cddr(sc->args)==sc->NIL) {
>               Error_0(sc,"block-set!: needs three arguments");
>          }
>          if(!isinteger(caddr(sc->args))) {
>               Error_1(sc,"block-set!: not an integer:",caddr(sc->args));
>          }
>          c=ivalue(caddr(sc->args))%255;
>
>          str[index]=(char)c;
>          s_return(sc,car(sc->args));
>     }

     Same for the predicate in opexe_3.

|     case OP_VECTORP:     /* vector? */
|          s_retbool(isvector(car(sc->args)));
>     case OP_BLOCKP:     /* block? */
>          s_retbool(ismemblock(car(sc->args)));
